In-app vault export support (#729)

* First pass at vault export UI

* Password validation via cryptoService

* Export service framework

* support for constructing json export data

* Support for constructing csv export data

* Cleanup and simplification

* Completion of vault export feature

* Formatting and simplification

* Use dialog instead of toast for invalid master password entry

src/App/Abstractions/IDeviceActionService.cs

@@ -13,6 +13,7 @@ namespace Bit.App.Abstractions
         Task ShowLoadingAsync(string text);
         Task HideLoadingAsync();
         bool OpenFile(byte[] fileData, string id, string fileName);
+        bool SaveFile(byte[] fileData, string id, string fileName, string contentUri);
         bool CanOpenFile(string fileName);
         Task ClearCacheAsync();
         Task SelectFileAsync();